The present invention relates to a method of producing dichloroethane (ethylene dichloride) (hereinafter referred to as "EDC") by reacting ethylene and chlorine in a liquid reaction medium. More particularly, the invention relates to a method of producing EDC in a high selectivity and a high efficiency with preventing the discharge gas from the reactor from forming an explosive mixture.
EDC is industrially important as a starting material in the production of vinyl chloride monomer. Various methods for producing EDC and improvements thereof have hitherto been proposed, for instance, as disclosed in U.S. Pat. No. 2,929,852, British Pat. No. 1,184,576, Japanese Patent Publication Kokai No. 57906/1973 and Japanese Patent Publication Kokai No. 177923/1983. In the production of EDC by direct chlorination, ethylene and chlorine are fed to the reaction system, and when ethylene is fed in a stoichiometrically excess amount, the unreacted ethylene is discharged.
Chlorine to be fed to a reactor has been produced by the mercury process. In recent years, in view of prevention of mercury pollution, the production of chlorine is being switched from the mercury process to the diaphragm process as an industrial chlorine production process not using mercury. This switching of the process has greatly altered the quality of chlorine gas. Oxygen gas included in the chlorine gas produced by the mercury process mainly results from incorporation of air, and the oxygen concentration is at most about 0.5%. In contrast, the chlorine gas produced by the diaphragm process contains about 1 to 5% of oxygen.
In another aspect, as disclosed in Japanese Patent Publication Kokai No. 177923/1983, it is known that addition of oxygen is preferable for efficiently producing EDC. In the production of EDC by addition reaction of ethylene and chlorine, trichloroethane is by-produced by chlorine substitution reaction to thereby lower the reaction yield. Oxygen can depress this side reaction. Therefore, even in the case where the amount of oxygen included in the fed chlorine gas is small, oxygen may be utilized in an amount up to about 10% based on the amount of fed chlorine by adding oxygen gas in view of the reaction yield.
The reaction of ethylene and chlorine to produce EDC is accompanied by heat generation of about 50 kcal/mole. In the past, the reaction was conducted at about 50.degree. C. with cooling with water or the like. Since a method which utilizes the heat of reaction efficiently was proposed in U.S. Pat. No. 2,929,852, there has been widely practiced a so-called high temperature method wherein ethylene is reacted with chlorine at a temperature not lower than 83.degree. C. by feeding the reactants into a liquid reaction medium composed mainly of EDC. In accordance with the method proposed in the above U.S. patent, a reaction-distillation scheme is adopted, namely the liquid reaction medium is vaporized by the heat of reaction and the vapor generated is introduced for purification into a distillation column connected with the upper part of the reactor, so that the heat of reaction can be efficiently utilized as a source of energy required for purification of EDC.
In the high temperature method, it is particularly effective to use oxygen in order to prevent by-production of trichloroethane by chlorine substitution side reaction, as disclosed in Japanese Patent Publication Kokai. No. 177923/1983. Also, in the high temperature method, for keeping the selectivity of reaction on a high level, it is effective to use ethylene in excess, as proposed in British Pat. No. 1,184,576. When ethylene is used in excess, unreacted ethylene is discharged. Therefore, it is important to recover the unreacted ethylene discharged. However, increase of the discharged unreacted ethylene to be treated for recovery in a second reactor decreases the utilization efficiency of the heat of reaction in the main reactor.
When chlorine gas containing a large amount of oxygen is fed to a reactor, most of the chlorine is consumed for the reaction, then oxygen is concentrated at a flow passage of exhaust gas discharged from the reactor, or at other places. Therefore, the risk that the discharge gas containing a high concentration of oxygen gas and an inflammable gas such as ethylene forms an explosive gas (detonating gas), greatly increases.
One of the safety measures which have hitherto been taken to counter the problem of explosion risk, is a method wherein a mixed gas is diluted by introducing an inert gas such as nitrogen to a flow passage where oxygen in the discharge gas from the reactor is concentrated, thereby avoiding formation of a detonating gas. However, this method has the disadvantages that a large amount of inert gas must be introduced for dilution and that it is difficult to recover ethylene included in the discharge gas, because of lowering of the ethylene concentration of the discharge gas to be fed to a second reactor for recovery.
As another safety measure, it is proposed, for instance, in Japanese Patent Publication Kokai Nos. 105603/1975 and 105604/1975 and Japanese Patent Publication Kokoku No. 6926/1985, to avoid formation of a detonating gas by adding ethylene to the discharge gas or by controlling the amount of ethylene to be fed to the reactor so as to raise the ethylene concentration of the discharge gas. This method has the disadvantages that it is necessary to discharge an amount of ethylene sufficient to avoid formation of an explosive mixture in the unreacted state in accordance with the amount of oxygen utilized for the reaction, thus usually ethylene must be recovered using a second reactor or the like, and that when the amount of oxygen is relatively large, the load on the second reactor becomes large relative to the main reactor.
It is an object of the present invention to eliminate the disadvantages of the conventional techniques applied to avoid formation of an explosive gas mixture in a passage of the discharge gas from a reactor for the production of EDC from ethylene and chlorine.
A further object of the present invention is to provide a process for producing EDC from ethylene and chlorine in high selectivity, yield and efficiency, wherein an explosive gas mixture is prevented from being formed in a passage of the discharge gas from a main reactor for the reaction of ethylene and chlorine, while in order to maintain the selectivity on a high level a relatively large amount of oxygen is utilized and an excess amount of ethylene is maintained on a proper level.
These and other objects of the present invention will become apparent from the description hereinafter.